STATION PRESSURE: This is the pressure that is observed at a specific
elevation and is the true barometric pressure of a location. It is the pressure
exerted by the atmosphere at a point as a result of gravity acting upon the
"column" of air that lies directly above the point. Consequently, higher
elevations above sea level experience lower pressure since there is less
atmosphere on which gravity can act. Put another way, the weight of the
atmosphere decreases as one increases in elevation. Consequently then, in general,
for every thousand feet of elevation gain, the pressure drops about 1 inch of
mercury. For example, locations near 5000 feet (about 1500 meters) above mean sea
level normally have pressures on the order of 24 inches of mercury.
ALTIMETER SETTING: This is the pressure reading most commonly heard in
radio and television broadcasts. It is not the true barometric pressure at a
station. Instead it is the pressure "reduced" to mean sea level using the
temperature profile of the "standard" atmosphere, which is representative of
average conditions over the United States at 40 degrees north latitude. The
altimeter setting is the pressure value to which an aircraft altimeter scale
is set so that it will indicate the altitude (above mean sea level) of the
aircraft on the ground at the location for which the pressure value was
determined. The altimeter setting is an attempt to remove elevation effects
from pressure readings using "standard" conditions.
MEAN SEA LEVEL PRESSURE: This is the pressure reading most commonly used
by meteorologists to track weather systems at the surface. Like altimeter setting,
it is a "reduced" pressure which uses observed conditions rather than "standard"
conditions to remove the effects of elevation from pressure readings. This reduction
estimates the pressure that would exist at sea level at a point directly below the
station using a temperature profile based on temperatures that actually exist at the
station. In practice the temperature used in the reduction is a mean temperature for
the preceding twelve hours. Mean sea level pressure should be used with caution
at high elevations as temperatures can have a very profound effect on the reduced
pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean
sea level pressure values.